There is a small post office in Mui Ne, east of the Ham Tien Market.
You are probably better to go to the large post office on Nguyen Tat Thanh St. in Phan Thiet. It is probably easier to send than receive. Packages received in Vietnam are highly taxed, regardless of the method they are shipped. Fedex is the most "reliable" and fastest method but still leaves a lot to be desired. Fedex will often drop off packages with neighbors or leave them at the provincial post office, rather than the specified address. If you use the postal service to send or receive, your package will be opened and thoroughly inspected. In Vietnam, the post office handles the phone systems, and all public phones are available there. Internet cafes now offer overseas calls as well.